[2][3][4] The design was built by the Clark Boat Company in Kent, Washington, United States, as well as eventually opening a facility in New Bern, North Carolina,[1] but it is now out of production.

[6] The San Juan 21 is a recreational keelboat, built predominantly of fiberglass with a balsa-cored cabin[1] and wood trim.

It has a fractional sloop rig, a raked stem, a slightly reverse transom, a transom-hung rudder controlled by a tiller and a centerboard keel.

[10] In a 2010 review Steve Henkel wrote, "the SJ21 has a very active racing association, unusual for a boat designed 30-plus years ago.

An efficient foil-shaped rudder and swing keel, plus a sailcloth slot gasket, offer superior hydrodynamics compared with her comps.